The present invention relates to a method of forming tubular members.

The present invention will be described with reference to the production of tubular frame members for automobiles, but should not be considered limited by use.

Frame members for automobiles are generally tubular since tubular members are light in weight whilst providing good mechanical properties.

In the usual method of manufacture of frame members the members are formed in two sections which are then welded along the edges thereof to provide a hollow member. Each member is generally stamped out from sheet metal.

The traditional method of producing the frame members involves numerous disadvantages, for instance the method is time consuming and the frame members produced by the method tend to have imperfections due to the welding operation. Also the stamping dies needed are quite costly.

In EP-A-0195157 there is proposed a method of making a box-section frame member from a metal tube. The method comprises the steps of first bending the tube to the general shape required for the frame member, then deforming the tube in a crimping operation using a first set of dies. The crimped tube member is then placed in a second die and internal pressure is applied to the tube member by the use of hydraulic fluid. The fluid pressure forces the tube member to take the shape of the die, which is the desired frame section. After application of the fluid pressure, the fluid is removed and the second die is

removed to leave the finished member.

In EP-A-0294034 a refinement of the method of EP-A-0195157 is disclosed. In EP-A-0294034, the need for a a first crimping die is dispensed with. Instead, a tube is taken and bent to the generally required shape and internal fluid pressure is then applied to the tube and a die closed around the tube. The die deforms the tube, with the fluid inside the tube acting to force the walls of the tube to take up the shape of the die. Once the die has been fully closed, the pressure of the fluid in the tube is then raised to deform the tube further to take up the required shape defined by the die. The fluid is then removed and the die opened to leave the finished member.

The present invention provides a method of forming a tubular member of desired cross-section from a tubular blank of a different cross-section by means of closing parts of a die around the tubular blank, the die defining a cavity having the desired cross-section, and supporting an interior surface of the tubular blank during closing of the parts of the die, characterised in that the interior surface of the tubular blank is supported during closing of the parts of the die by a deformable solid insert which is deformed when the tubular blank is deformed by the closing parts of the die and which resists deformation thereby providing support for the interior surface.

The novelty of the claimed method lies in the use of a solid insert instead of a fluid medium. This has the advantage that pressurising means for applying internal pressure to the blank are not needed; instead the solid insert applies the resistive force when the

die sections are closed and the blank is crushed. Therefore there is only a need to control external pressure applied on the blank by the die parts and there is no need to actively apply internal pressure.

The resilient solid insert can be of elastomeric material. An elastic resilient material is preferably used for the solid insert. In preferred embodiments a silicone rubber insert is used.

The use of an elastic resilient member has two main advantages. First, an elastic resilient material will apply internal pressure in proportion to the degree of deformation of the material. In this instance, the solid elastic resilient material will apply the greatest internal pressure on the tube member where most required, in the regions of greatest deformation. This contrasts sharply with the fluid, which applies the same pressure everywhere.

The second advantage of using an elastic resilient solid insert is that the solid insert can be reused in successive forming operations.

In one method according to the invention the solid insert is a thermoplastic and the method additionally includes the step of heating the formed tubular frame to melt the thermoplastic. This allows easy removal of the insert from the blank.

In another method according to the invention a solid insert is formed within the blank by mixing two liquids in the blank to form a polymeric solid.

Preferably a blank is used which has a cross-section which is uniform along the length thereof and the formed frame member has a cross-section which varies along the length thereof.

An additional step for the method is the step of applying pressure to the ends of the solid insert when

the parts of the die have been closed and whilst the insert is located within the blank.

Mechanical actuators can be used to pressurise the ends of the solid insert.

Preferably end caps are used to restrain expansion of the solid insert during the closing of the parts of the die.

Means can be provided to apply pressure to the solid insert during the closing of the parts of the die, for instance electro-mechanical or electro- hydraulic actuators can be used and a computer can be used to control the pressure applied to the solid by the electro-mechanical or electro-hydraulic actuators. This enables control of the resistance to deformation offered by the solid insert.

Preferably the tubular blank is bent to conform to the desired overall shape of the formed frame member before the parts of the die are closed around the tubular blank.

In a second aspect the present invention provides a method of forming a tubular member of a desired cross-section from a tubular blank of a different cross-section by means of, closing parts of a die around the tubular blank, the die defining a cavity having the desired cross-section, and applying an outwardly directed force on an interior surface of the tubular blank during closing of the parts of the die, characterised in that a deformable solid insert is located within the blank whilst the parts of the blank are closed around the blank, the, blank and the solid insert being deformed by the closing parts of the die and the solid insert resisting deformation and

thereby applying an outwardly directed force on the interior surface.

In a third aspect the present invention provides a method of forming a tubular member of a desired cross-section from a tubular blank of a different cross-section by means of closing parts of a die around the tubular blank, the die defining a cavity having the desired cross-section and the blank being deformed on closing of the die, characterised by providing a defor able solid insert within the blank during deformation of the blank, the solid insert deforming during deformation of the blank and absorbing energy during deformation.

Preferred methods according to the invention will now be described with reference to the accompanying drawings in which;

Figure 1 shows a tubular blank suitable for use in the method of the invention;

Figure 2 shows the tubular blank of Figure 1 with a deformable solid insert.

Figure 3 shows the tubular blank and solid insert of Figure 2 positioned between the open sections of the die.

Figure 4 shows in cross-section the die sections of Figure 2 closing in on the tubular blank and the solid insert.

Figure 5 shows the die sections brought together

around the tubular blank and the solid insert.

Figure 6 shows the frame member formed by the method.

Figure 7 shows an additional preparatory step for the method.

Figure 8 shows an additional finishing step which can be used in the method.

Referring to figure 1 there can be seen a steel tubular blank 10. The blank 10 is of uniform circular cross-section.

In the method, a solid insert 20 is inserted in to the tube, as seen in figure 2. The solid insert will typically be of silicone rubber and will be manually inserted into the blank 10. The insert will extend throughout a blank when the whole of the blank is to be deformed. When only a portion of the blank is to be deformed then the insert could be positioned in that portion.

The blank 10 is then bent using conventional methods and conventional pipe bending apparatus so that the circular tubular blank generally assumes the outline required for the frame member formed by the method, as can be seen in figure 3.

The shaped tubular blank 10 with the silicone rubber insert is then placed between the open sections 30 and 40 of a die, as can be seen in figure 4. The die sections 30 and 40 have matched recesses 31 and 41 on opposing surfaces. The matched recesses 31 and 41 define a cavity when the die sections 30 and 40 are brought into contact with each other. The end

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portions of each of the die sections 30 and 40 are semi-circular in cross-section (end portions 42 and 43 can be seen in figure 4) . The end portions extend only a little way into the die.

As can be seen in figure 4, the shaped tubular blank 10 is of a shape which generally conforms to the outline of the recess 41. The tubular blank 10 rests in the recess 41.

In the next step of the method the two die sections 30 and 40 are brought together by suitable mechanical means known in the art.

The die sections 30 and 40 exert a crushing pressure on the tubular blank 10, which deforms, as can be seen in figure 5. The deformation of the tubular blank 10 is resisted by the silicone rubber insert 20. The silicone rubber insert 20 resists the deformation to different degrees along the blank 10, the silicone rubber insert inserting a greater internal force on the tube 10 where deformation of the tube 10 is greater.

The resistance to deformation provided by the silicone rubber exerts on the blank 10 a radially outwardly directed force which prevents buckling of the blank 10 under the crushing pressure of the dies 30 and 40. The resilient silicone rubber insert acts to force the sides of the tubular blank 10 out towards the corner portions of the dies 30 and 40 and acts to urge the blank 10 to take up the cross-sectional shape of the cavity in the die. Without the insert the blank 10 would buckle under compression and would not assume the desired cross-section.

Eventually, as can be seen in figure 6, when the dies 30 and 40 are brought into engagement, the cross-section of the tube 10 is defined by the

interior of the cavity defined by the die sections 30 and 40.

End caps (not shown in the figures) may be used in abutment with the ends of the solid insert to restrain expansion of the insert along the blank 10. The circular end sections of the die leave the blank 10 with undeformed regions which can accomodate end caps.

When the blank 10 has been shaped by the die to the required cross-section, the die sections are opened and the formed tube removed. The silicone ruober insert will typically be removed from the blank 10 either whilst the blank 10 is located in the die frame or after removal of the blank 10 from the die, to save weight in the tube member and also so that the silicone rubber can be reused.

The circular end sections of the frame member are cut off when the frame member is removed from the die. The final finished frame member is shown in Figure 7.

In a variation on the above method, a solid insert is formed in the tube 10 by mixing of two liquids within the tube to form a solid polymer. This is illustrated in figure 8, where a tubular blank 50 is shown which is sealed at one end by an end cap 51. Two fluids are mixed within the tube 50 after being delivered by suitable fluid means 52 and 53.

A possible additional stage to the method is shown in figure 9. This stage will take place after the dies 30 and 40 have been brought together around the blank 10, as shown in figure 6. In the additional stage of figure 9, two actuators 60 and 61 are provided and apply pressure on the resilient silicone rubber solid insert 20 via suitable cylindrical bungs, one of which is shown at 62. The application of

pressure on the silicone rubber solid insert causes the insert to exert an outward radial force on the blank and ensures that the blank 10 fully takes up the shape of the cavity defined by the die sections 30 and 40. The bungs can move in the constant cross-section circular end regions of the blank 10 in the die, which remain undefor ed when the parts of the die are brought together.

Whilst in the methods described above a silicone rubber insert 20 is used, numerous materials could be used for this purpose. An important factor in determining suitability of a material is its bulk modulus. The silicone rubber used in the preferred method has a bulk modulus which corresponds by and large to the bulk modulus of hydraulic fluid. The use of silicone rubber material for the solid insert of the method is especially advantageous since the properties of silicone rubber allow easy removal of the substance from the formed tubes. A silicone rubber solid insert once removed regains its original shape and can be used in successive forming operations, typically lasting for around 100 forming operations. The silicone rubber used will typically be of the RTV (Room Temperature Vulcanising) grade of silicone rubber, which in the U.K. can be obtained from Wacker Chemicals Ltd.

Instead of the silicone rubber, a thermoplastic material could be used and this has the advantage that the thermoplastic could be melted out of the tube, once the tube has been formed. Suitable Thermoplastics are VINAMOLD and GELFLEX (proprietary marks) which are available from K & C Mouldings of Shelfanger, Diss, Norfolk.

The method of the invention is simple to operate

and therefore is ideally suited for the manufacture of prototype frame members.

Whilst the tubular blank 10 used in the preferred embodiment is made out of steel, the tube could be of any metal. Aluminium is commonly used for vehicle frames. The method could also be used with non-metallic materials, provided that the materials have properties suitable for the process.

The solid insert for the tube is preferably inserted in the tube without precompression, so that the insert is first compressed when the tube is compressed.

The bending methods for initially bending the circular tube to the general outline shape of the required frame member are well known in the art and include mandrel bending.

The applicant envisages an improved more complex variation of the above described method. In such a method, actuators would be used to act via bungs on the ends of a solid insert in a blank whilst the blank is deformed. The bungs will be movable in the ends of the blank which remain of constant cross-section during deformation.The actuators could be computer controlled to vary the resistance offered to crushing of the blank in accordance with a programme provided by the operator. Thus, the operator can control precisely the degree of resistance to deformation offered by the solid insert.

It is preferable to use a homogeneous substance as the solid insert.

The term solid insert used in the specification and claims includes a hollow insert of solid material, which could be used in some instances.

Whilst in the above embodiment the insert is

within the blank before it is initially shaped, the solid insert could be located in the blank after it is bent to the general outline shape.

Whilst in the above embodiment the blank used is of constant circular cross-section, any hollow blank of any cross-section could be used. It should also be appreciated that the method could be used to form both frames of constant cross-section and frames of varying cross-section.

It must also be understood that whilst it is preferable to provide end region portions in the die whch do not deform the blank, this is not an essential feature of the invention, but is preferable to allow easy use of bungs and end caps. Nevertheless the applicant envisages the possible use of a die which deforms a blank along the entire length thereof.

It will be understood that the present process provides a forming method quite different from the prior art since the material inside the tube provides a resistive force acting against the force of the dies, rather than applying a pressure in its own right.

Whilst the method of the invention has been described in the specific embodiment by reference to a method of forming frame members for automobiles, the method of the invention can be used to manufacture any structural member. The method can also be used to manufacture pipes which have no structural supporting function, eg. coolant pipes.

The method provided by the invention is a lot simpler than the forming methods of the prior art, since it does not require complex fluid seals or apparatus for applying fluid pressure. The method is also suitable for forming frame members from tubular blanks which have apertures extending through the blanks. Obviously, fluid would not be suitable for use with such material, due to problems of leakage.